The present invention relates to the production of ferromolybdenum alloy useful as a furnace additive. In particular, the present invention relates to the production of a ferromolybdenum alloy having a molybdenum content of at least 50% by weight and a carbon content of not more than 0.1% by weight with a high recovery of molybdenum, desirably in the order of at least 96% by weight, in which process a molybdenum oxide concentrate is reduced in a molten iron containing slag bath in a furnace using a carbonaceous reducing agent for reducing the concentrate.
Heretofore, ferromolybdenum has been commercially produced from a molybdenum oxide concentrate by means of the thermite or ferrosilicon reduction process to ASTM Specification A132 (60% molybdenum minimum, 0.1% carbon maximum, 0.05% phosphorous maximum, 0.15% sulphur maximum, 1.0% silicon maximum, 1.0% copper maximum, 0.01% lead maximum and 0.01% tin maximum). However, ferrosilicon as a reductant in a thermite process is an expensive material, its supply is becoming more limited and the production of high grade ferrosilicon for use in the process is inefficient electrically and involves a substantial expenditure in energy.
Attempts have previously been made to produce ferromolybdenum in an electric arc furnace using carbon as the reductant as this is usually a plentiful material and is inexpensive. Examples of such attempts are given in Elyutin et al "Production of Ferro-Alloys: Electrometallurgy", translated by T. R. Shapiro, pp. 256-287, National Science Foundation, Washington, D.C. 1961 and B.I.O.S. Final Report No. 798 "The German Ferro-Alloy Industry", London, December, 1945. However, such procedures have heretofore produced only high carbon, high silicon, low molybdenum ferromolybdenum alloys with relatively poor overall molybdenum recoveries. In particular, in one of the procedures set forth in the aforesaid Elyutin publication, ferromolybdenum was produced from roasted molybdenum oxide concentrate by carbon reduction in a low power single phase electric furnace lined with coal. In the procedure the calcined concentrate was briquetted with coal dust and fed into the electric furnace adding at the same time, lime and iron. Smelting was carried out by fusing on the block at low temperature. The alloys obtained are stated to have in a number of cases a high carbon content and are resmelted and decarburized.
In another procedure, molybdenum has been smelted using a carbon containing reducing agent in a small single phase 300 to 500 kw and a three phase 500 to 1500 kw electric furnace in which the wall lining is rammed dolomite, and the hearth is coal. In such a method the charge consists of briquettes of roasted molybdenum concentrate with charcoal or peat coke. Powdered lime, iron powder and other materials are used as binder. The block waste and the slag of previous smelting are melted first, then briquettes of the conventional composition are added gradually. After sufficient quantity of the metal has been formed on the hearth, briquettes with an insufficient content of reducing agent are fed to the furnace in order to reduce the carbon content. During the smelting period a certain quantity of iron was added to the bath. The alloy produced by such a process contained 35 to 50% molybdenum, 1 to 2% silicon and 6 to 8% carbon. Such alloys are totally unsatisfactory in that they do not in any way meet the above ASTM Specification as set forth above and further, the molybdenum recovery in the process is low and renders these processes commercially unviable.